1. Field of the Invention
The present invention relates to an inertia force balancing apparatus for a machine provided with a reciprocating heavy object, such as a Pilger rolling mill for producing seamless steel pipes and the like.
2. Description of the Prior Art
Generally, in a Pilger rolling mill in which Pilger mill rolls are installed, as shown in Japanese Pat. Publication No. 43472/76, a material pipe is rolled between a pair of Pilger mill rolls having special caliper compasses and a mandrel rod so as to produce seamless steel pipes. To give a description of its arrangement and operation with reference to a schematic diagram of a conventional example as shown in FIG. 2, a crankarm 3 and a fan-shaped balancer 4 are secured to a crankshaft 2 which is rotated by a main motor. A control rod 5 and a connecting rod 6 for a V-balancer are respectively connected to the crankarm 3. A Pilger mill stand 8 in which a pair of Pilger mill rolls are mounted is connected to the distal end of the connecting rod 5, and a V-balancer 9 is suspended from the distal end of the connecting rod 6.
When the crankshaft 2 is rotated by the main motor at a fixed rotational speed .omega., the Pilger mill stand 8 reciprocates via the crankarm 3 and the connecting rod 5. In conjunction with the reciprocal motion thereof, the Pilger mill rolls 7 are rotated by means of a rack and pinion (neither are shown) to roll a pipe being worked. More specifically, when the Pilger mill rolls 7 rotate and the pipe with the mandrel rod inserted therethrough advances, the rolls 7 bite into the pipe, and when the rolls 7 further rotate, the pipe is rolled down to a finishing dimension. Subsequently, the pipe is released from the rolls 7. Although the pipe remains stationary while being rolled by the rolls 7, the pipe advances simultaneously as it is freed from the rolls 7.
With the Pilger rolling mill having the above-described arrangement, a crank mechanism is adopted to reciprocate the Pilger mill stand 8 with the Pilger mill rolls 7 mounted therein. Hence, imbalance is created as a result of the inertia force of the reciprocal motion which is induced by the cranking motion, as well as a couple caused by the inertia force. Accordingly, in order to eliminate this imbalance, the fan-shaped balancer 4 and the V-balancer 9 are installed, as described above.
With the conventional Pilger rolling mill, however, the following drawbacks occur owing to the installation of the fan-shaped balancer 4 and the V-balancer 9: (1) The overall apparatus becomes large in size due to the installation of the fan-shaped balancer 4 and the V-balancer 9. (2) With the fan-shaped balancer 4 and the V-balancer 9, although it is possible to eliminate the imbalance of a term of first degree (this refers to a term of first degree in a known general formula expressing a inertia force of reciprocal motion; hereafter the same) based on the rotational speed .omega. of the crankshaft 2, the imbalance of the term of higher degree cannot be eliminated. If an attempt is made to reduce the imbalance of the term of higher degree, the ratio of the length R of the crankarm 3 to the length L of the connecting rod 5 must be decreased, and this results in the length L of the connecting rod 5 increasing, the overall apparatus hence becoming large in size. (3) Since the imbalance of the term of higher degree based on the rotational speed .omega. of the crankshaft 2 cannot be eliminated, a large inertia force proportional to the raising of .omega. to second power is received by the connecting rod 5, the crankshaft 2 and the like. Therefore, if the speed is increased, provision of a structure which is capable of withstanding this inertia force becomes unrealistic, and there are, hence, limitations to the amount by which the speed can be increased. (4) Due to the provision of the V-balancer 9, a Pilger rolling mill employed for rolling, for instance, a material steel pipe with a 260 mm diameter requires about 8 m-deep foundation work. In the final analysis, it would be difficult to provide adequate maintenance for the V-balancer 9.
On the other hand, in a Pilger rolling mill in which a Pilger mill stand with a pair of Pilger mill rolls is mounted, a method of balancing an inertia force is known wherein a piston rod of an air cylinder is connected to the Pilger mill stand, and compressed air is supplied to the air cylinder, thereby balancing the inertia force of the reciprocating Pilger mill stand (see the specification of U.K. Pat. No. 1355733).
However, with such a known method of balancing the inertia force, there have naturally been problems in that, since the inertia force is balanced by compressed air whose volume can be diminished, the compressor and the air cylinder for the compressed air become large in size, and that, even if an attempt is made to back up the force of the Pilger mill roll to roll a pipe by means of the compressed air, the compressed air is unable to provide a powerful force. Moreover, if an attempt is made to balance the inertia force of the reciprocal motion by driving the air cylinder by compressed air, excessively large energy is required as a driving force for this purpose.